Engine Temperatures
The diagram shows the temperature-entropy
plot for an Ideal Otto cycle.
Temperature (1) is the inlet gas temperature,
T(2) is the temperature after the gas is adiabatically compressed
in the compression stroke, T(3) is the temperature reached by the gas after
the constant volume "combustion" process, T(4) is the result of the adiabatic
expansion during the work stroke.
The removal of exhaust heat at constant
volume returns the system to its initial temperature.
From: Cengel & Boles, "Thermodynamics" McGraw Hill (1989)